Do you tell your ex-girlfriend's parents that she is dating a married man?

No. Absolutely not. It is not your place to interfere in your ex girlfriends relationships. It is her responisibility morally to stop having a relationship with this man and if she chooses not to then it is her choice on who she chooses to let in on her secret. As a friend you many voice your opinion to the friend but that is as far as it goes. Be a friend until the end then be the support.